Anodizing is an example of a general class of coating known as conversion coatings, in which the metal surface is converted into the coating by means of a chemical or electro-chemical process. Other examples of conversion coatings include chromate conversion coatings, phosphate conversion coatings, bluing, and oxide coatings on steel. They are used for corrosion protection, increased surface hardness, to add decorative color, and as paint primers. Conversion coatings can be very thin, on the order of 0.0001 inches (approximately 2.5 microns). Thicker coatings, up to 0.002 inches (approximately 50 microns), are usually built up on aluminum alloys, either by anodizing or chromate conversion. The present disclosure is designed to be used in conjunction with an anodized conversion coating.
Anodizing (or anodising) is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of a metal part. Natural oxides are typically rough, irregular, and not continuous, whereas the anodized oxide layer is more uniform. The process derives its name from the fact that the part to be treated forms the anode portion of an electrical circuit in this electrolytic process. The anodizing process is of little use on carbon steel because the iron oxide formed by the process (rust) puffs up and flakes off, constantly exposing new metal to corrosion. But on many other metals the oxide forms as a tightly adhering layer, so that anodizing increases corrosion resistance and wear resistance, and provides a better substrate for adhesion of secondary layers such as paints, primers, and glues than does the bare metal.
Anodic films are most commonly formed to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, niobium and other metals and alloys. On all these metals, anodic films are generally much stronger and more adherent than most paints and platings, making them less likely to crack and peel. Anodization changes the microscopic texture of the surface and can change the crystal structure of the metal near the surface. Anodic coatings are often porous, thick ones inevitably so. For example, chromic acid and sulfuric acid anodizing processes, commonly referred to as Types I and III, produce pores in the anodized coat. The thick porous anodic coatings can be useful for creating cosmetic effects, because they can absorb dyes. On the other hand, the porous nature of the coating limits the corrosion resistance, and thus a sealing process that fills the pores is often used to improve corrosion resistance and reduce the tendency for staining. However, the sealing process is generally expensive and time consuming. Long immersion in boiling deionized water is the simplest sealing process, although it is not completely effective and reduces abrasion resistance by approximately 20%. Teflon, nickel acetate, cobalt acetate, and hot sodium or potassium dichromate solutions are also commonly used as sealants. The best sealants are chromate-based chemicals that are very toxic and environmentally hazardous. Furthermore, for many applications anodizing and sealing does not provide sufficient protection against corrosion. This is especially true when a very thin anodized layer is used for the purpose of retaining the bright polished appearance of the metal substrate. Such a polished appearance is desirable as a decorative finish and is also required for many applications such as light reflectors.
In review, anodizing is a widely used and effective method of generating a thin, hard, protective coating on metals such as aluminum, titanium, zinc, magnesium, and niobium. The resulting coatings are all porous to some degree, which improves the adhesion of secondary coatings such as paints and which is useful for holding dyes used to color the surface. However, the porous nature of anodized coatings limits their corrosion protection, and thus they are often sealed to close the pores. Presently available sealing treatments are not entirely satisfactory in terms of effectiveness at preventing corrosion, cost, stain resistance, heat resistance, UV resistance, and environmental impact. Thus there is a need for an alternative means of sealing or otherwise improving corrosion resistance and other characteristics of anodized coatings.